A popular form of recreational watercraft is the inflatable boat. Conventional designs in this category have an inflatable horseshoe-shaped hull member defining the bow and sides of the boat, and a generally flat floor attached to the inflatable horseshoe. The stern of the boat may be an additional inflatable member or a rigid transom to receive an outboard motor for mounting. These boats are typically made of layered materials, one layer to provide an air/watertight seal and another to provide shape, strength and some measure of abrasion resistance. These watercraft allow easy land transportation, being lightweight and foldable into small packages upon deflation. They also are extremely buoyant, allowing use in rough waters or surf and provide added safety for inexperienced recreational users.
This conventional design does have some disadvantages, however. Abrasion limits the life of these craft. Beaching, launching, debris in water and cleaning of the external flexible surfaces tend to abrade the material which leads to failure and/or loss of buoyancy. This type of boat is also frequently used in shallow areas, where bottom scraping is likely, further aggravating the abrasion problem.
A second disadvantage is the limitations on speed. The nonrigid form of the inflatable boat tends to deform under high speed loads. The high buoyancy and non-rigid form also tends to ride on and deform to the shape of the tops of waves, slowing the craft even if streamlined. Maintaining proper streamlined shape under these conditions is difficult at best with an inflatable boat. The higher propulsive forces (outboard motor) for high speed must also be accommodated and distributed along the inflated boat. These forces tend to result in a heavier, less flexible (foldable) structure if the conventional design is continued. The combination of added weight and lack of streamlining requires still heavier and more powerful propulsive means, which further increases weight and decreases portability and economy.
Another disadvantage is the general lack of directional stability. Rigid boats typically have a keel from bow to stern. The keel provides the primary structure to transfer fore/aft loads and provide directional stability against side winds and loads. Inflatable boats lack this rigid load carrying member and directional stability.
In inflatable boats which do not come equipped with a rigid transom, a demountable transom is sometimes provided for attachment to the rear portion of the inflatable fabric surface, for mounting of an outboard motor. A disadvantage with this type of installation is that at high power settings the outboard motor will tend to deform the inflatable supporting structure and drive itself under the boat, limiting speed and efficiency of propulsion. In the instant invention, an additional rigid supporting surface is provided to support the demountable transom, and provide more rigid support for the outboard motor preventing deformation and improving efficiency.
Modifications of inflatable boats have addressed some of these disadvantages. U.S. Pat. No. 3,451,078 teaches that rigid boards can be fitted into the inflatable hull by means of waterproof joints. Boards or joint sections are placed in areas of high loads, or where abrasion resistance and dimensional rigidity are needed. Boards may also provide a keel for the boat. This modification (sealed board sections) allows more limited folding of deflated boat, but adds weight and limits transport size to the board sections. However the major disadvantage is the cost penalty incurred. The waterproof seal required for each board edge and board joint requires additional manufacturing process steps and therefore added cost. The areas between board sections must also be increased in strength and abrasion resistance to allow concentrated and repeated folding, and to transfer loads from one board to another.
Another modification also discussed in U.S. Pat. No. 3,451,078 uses bottom boards fitted above the inflatable hull. These again provide load bearing means, especially in conjunction with outboard motor transoms.
Another type of modification is represented by U.S. Pat. No 4,462,331. This teaches a modified layered construction of inflatable boats to add rigidity and avoid blisters. However, cost and weight penalties are present.
A more drastic design modification changes the inflatable into a semi-rigid boat, as represented by U.S. Pat. No. 4,660,497. A rigid single hull portion is continuously or frequently attached to a number of inflatable bladders below an inflatable boat. This converts the inflatable boat into a rigid bottom craft, which also allows a fixed or hinged attachment of rigid bottom to a transom for motor mounting. Loads can be transmitted, shape maintained, and directional stability achieved. However, the craft is no longer foldable (single section bottom) or fully deflatable. The single rigid hull portion also adds weight. The single hull section may include flotation chambers to aid stability but this increases transport difficulties. In the extreme form of this design, the boat is essentially a rigid hull craft with inflatable bladders.
Another approach is to start with a rigid watercraft and add flexible floats or protectors. This is illustrated in the following U.S. Pat. Nos: 4,667,619; 3,467,345; 3,055,022; 3,220,026; 4,586,451; and 3,270,701. This approach adds relatively lightweight stabilizers, protectors and floats to a rigid boat (or surfboard). However, ease of transport, folding, light weight and other advantages of inflatable boats are lost.